Antibodies Against ARHGDIB and ARHGDIB Gene Expression Associate With Kidney Allograft Outcome

Early and late antibody mediated rejection: Which game is the complement playing?

The role of the complement system in antibody mediated rejection (AMR) emerged in the last decades, and the demonstration of the presence of complement fragments in renal allograft biopsies is a consolidated diagnostic sign of AMR. However, antibodies against donor antigens may lead to microvascular inflammation and endothelial injury even in the absence of complement activation, and growing evidence suggests that complement-independent mechanisms may be prominent in late (i.e., occurring >6 months after transplantation) vs early AMR. Different donor specific antibodies (DSA) with different biological features and complement activation ability may be involved in late or early AMR. Downregulation of tissue complement inhibitors may happen early after transplantation, partially due to ischemia reperfusion injury, and could facilitate complement activation in early vs late AMR. Clinical and histological features of late AMR and C4d negative AMR seem to converge, and this narrative review analyzes the evidence that supports lower complement activation in late vs early AMR, including differential C4d staining prevalence based on the time after transplantation, differential response to anti-complement therapy and other direct and indirect signs of the complement system activation. The therapeutic approach in early vs late AMR should take into account possible differences in the pathophysiological mechanisms of microvascular inflammation and endothelial injury in early vs late AMR.

Novel Aspects of Immunogenetics and Post-Transplant Events in Kidney Transplantation

HLA typing and matching have been crucial in kidney transplantation, but methods for assessing tissue histocompatibility have advanced significantly. While serological-level HLA typing remains common, it captures only a small fraction of true HLA variation, and molecular matching is already replacing traditional HLA matching. Recent studies have expanded our understanding of genetic tissue compatibility beyond HLA loci. Candidate gene analyses and genome-wide association studies (GWAS) have identified genetic factors linked to post-transplant complications, though replication of these findings is challenging. An alternative approach involves genome-wide matching of genes or genetic variations. This method has shown promise in hematopoietic stem cell and kidney transplantation. For instance, homozygous gene deletions in LIMS1 or complement factor H (CFH) genes have been associated with acute rejection risk. This may be due to alloimmune responses against proteins absent in the patient but present in the graft, or due to the missing protein's function. Genetic studies in clinical medicine face challenges due to the interplay of genetic and environmental factors, necessitating large datasets for meaningful associations. International collaboration and large consortia, like iGeneTRAin, are essential for validating findings and advancing the field. This review highlights recent advancements in immunogenetics and tissue histocompatibility, emphasizing future research directions.

Comprehensive Characterization of Anti-HLA and Non-HLA Antibodies in Patients on Kidney Transplant Waiting List and Evaluation of Their Impact on Alloimmunization Risk and Dialysis Treatment

Alloimmunization remains a major obstacle to successful kidney transplantation, mainly due to the formation of anti-HLA antibodies. In recent years, non-HLA antibodies have emerged as additional immunologic factors that can potentially contribute to graft rejection. The aim of this study was to investigate the prevalence and specificity of both anti-HLA and non-HLA antibodies in patients with end-stage renal disease on a waiting list for kidney transplantation. Serum samples from 74 patients were analyzed using complement-dependent cytotoxicity and solid-phase assays. IgG anti-HLA antibodies were identified in 43.2% of participants, while IgG non-HLA antibodies were detected in 91.9%. The most frequent non-HLA antibodies included anti-ENO1 (28.4%), anti-FIBR1 (23.0%) and anti-PRKCZ (23.0%). A significant difference was found between the number of distinct IgG anti-HLA and IgG non-HLA antibody specificities. However, no significant correlation was found between the number of IgG non-HLA antibody specificities and previous alloimmunization events or dialysis treatments. These results suggest that non-HLA antibodies, although often overlooked, can sometimes play a critical role in transplant outcomes. Routine testing for non-HLA antibodies, in addition to mandatory anti-HLA antibody screening and identification, could improve immunologic risk assessment in transplant patients and post-transplant care.

Report of the 2022 Banff Heart Concurrent: Focus on non-human leukocyte antigen antibodies in rejection and the pathology of "mixed" rejection

The Banff Heart Concurrent Session, held as part of the 16th Banff Foundation for Allograft Pathology Conference at Banff, Alberta, Canada, on September 21, 2022, focused on 2 major topics: non-human leukocyte antigen (HLA) antibodies and mixed rejection. Each topic was addressed in a multidisciplinary fashion with clinical, immunological, and pathology perspectives and future developments and prospectives. Following the Banff organization model and principles, the collective aim of the speakers on each topic was to • Determine current knowledge gaps in heart transplant pathology • Identify limitations of current pathology classification systems • Discuss next steps in addressing gaps and refining classification system.

Chronic lung allograft dysfunction is associated with an increased number of non-HLA antibodies

BACKGROUND

Chronic lung allograft dysfunction (CLAD) is the major cause of adverse outcomes in lung transplant recipients. Multiple factors, such as infection, alloimmunity, and autoimmunity, may lead to CLAD. Here, we aim to examine the role of non-human leukocytes antigen (HLA) antibodies in CLAD in a large retrospective cohort.

METHODS

We analyzed non-HLA antibodies in the pre- and post-transplant sera of 226 (100 CLAD, 126 stable) lung transplant recipients from 5 centers, and we used a separate cohort to confirm our findings.

RESULTS

A panel of 18 non-HLA antibodies was selected for analysis based on their significantly higher positive rates in CLAD vs stable groups. The panel-18 non-HLA antibodies (n > 3) may be positive pre- or post-transplant; the risk for CLAD is higher in the latter. The presence of both non-HLA antibody and HLA donor-specific antibody (DSA) was associated with an augmented risk of CLAD (HR=25.09 [5.52-14.04], p < 0.001), which was higher than that for single-positive patients. In the independent confirmatory cohort of 61 (20 CLAD, 41 stable) lung transplant recipients, the risk for CLAD remained elevated in double-positive patients (HR=10.67 [0.98-115.68], p = 0.052). After adjusting for nonstandard immunosuppression, patients with double-positive DSA/Non-HLA antibodies had an elevated risk for graft loss (HR=2.53 [1.29-4.96], p = 0.007).

CONCLUSIONS

Circulating non-HLA antibodies (n > 3) were independently associated with a higher risk for CLAD. Furthermore, when non-HLA antibodies and DSA were detected concomitantly, the risk for CLAD and graft loss was significantly increased. These results show that humoral immunity to HLA and non-HLA antigens may contribute to CLAD development.

Transplant Candidates of 70+ Years Have Superior Survival If Receiving Pre-Emptively a Living Donor Kidney

Background: The number of kidney transplant recipients over 70 years of age is increasing but detailed data on patient and graft survival in the modern era of immune suppression are few. Methods: A single-center cohort of patients of 70 years and older (n = 349) at time of kidney transplantation from 2010-2020 were followed until January 2023. Results: The median age was 73 years with a median follow-up of 4.3 years. Fifty percent of recipients of a living donor kidney (LDK, n = 143) received their graft pre-emptively. Cumulative death-censored graft survival was excellent in the LDK group and reached 98% at 5 years vs. 85% in the deceased donor kidney (DDK) group. Primary non-function (38%) and rejection (43%) were the major causes of graft loss in the first year after DDK transplantation. Rejection-related graft loss was 4.6% during follow-up. Median recipient survival was superior in the subgroup of pre-emptively transplanted LDK patients compared to non-pre-emptively LDK transplanted patients (11.1 versus 6.2 years). Non-pre-emptively transplanted patients had a significantly increased incidence of infection (HR 3.81, 1.46-9.96) and cardiovascular-related causes of death (HR 3.35, 1.16-9.71). Pre-emptive transplantation was also associated with a significantly improved graft survival in the DDK recipients but this result was confounded by significantly better HLA matching and younger donor age in this group. Conclusions: Pre-emptive LDK transplantation in patients of 70 years or older confers superior graft and recipient survival.

The role of anticomplement therapy in the management of the kidney allograft

As the number of patients living with kidney failure grows, the need also grows for kidney transplantation, the gold standard kidney replacement therapy that provides a survival advantage. This may result in an increased rate of transplantation from HLA-mismatched donors that increases the rate of antibody-mediated rejection (AMR), which already is the leading cause of allograft failure. Plasmapheresis, intravenous immunoglobulin therapy, anti-CD20 therapies (i.e., rituximab), bortezomib and splenectomy have been used over the years to treat AMR as well as to prevent AMR in high-risk sensitized kidney transplant recipients. Eculizumab and ravulizumab are monoclonal antibodies targeting the C5 protein of the complement pathway and part of the expanding field of anticomplement therapies, which is not limited to kidney transplant recipients, and also includes complement-mediated microangiopathic hemolytic anemia, paroxysmal nocturnal hemoglobinuria, and ANCA-vasculitis. In this narrative review, we summarize the current knowledge concerning the pathophysiological background and use of anti-C5 strategies (eculizumab and ravulizumab) and C1-esterase inhibitor in AMR, either to prevent AMR in high-risk desensitized patients or to treat AMR as first-line or rescue therapy and also to treat de novo thrombotic microangiopathy in kidney transplant recipients.

Defining a natural killer cell-enriched molecular rejection-like state in lung transplant transbronchial biopsies

In lung transplantation, antibody-mediated rejection (AMR) diagnosed using the International Society for Heart and Lung Transplantation criteria is uncommon compared with other organs, and previous studies failed to find molecular AMR (ABMR) in lung biopsies. However, understanding of ABMR has changed with the recognition that ABMR in kidney transplants is often donor-specific antibody (DSA)-negative and associated with natural killer (NK) cell transcripts. We therefore searched for a similar molecular ABMR-like state in transbronchial biopsies using gene expression microarray results from the INTERLUNG study (#NCT02812290). After optimizing rejection-selective transcript sets in a training set (N = 488), the resulting algorithms separated an NK cell-enriched molecular rejection-like state (NKRL) from T cell-mediated rejection (TCMR)/Mixed in a test set (N = 488). Applying this approach to all 896 transbronchial biopsies distinguished 3 groups: no rejection, TCMR/Mixed, and NKRL. Like TCMR/Mixed, NKRL had increased expression of all-rejection transcripts, but NKRL had increased expression of NK cell transcripts, whereas TCMR/Mixed had increased effector T cell and activated macrophage transcripts. NKRL was usually DSA-negative and not recognized as AMR clinically. TCMR/Mixed was associated with chronic lung allograft dysfunction, reduced one-second forced expiratory volume at the time of biopsy, and short-term graft failure, but NKRL was not. Thus, some lung transplants manifest a molecular state similar to DSA-negative ABMR in kidney and heart transplants, but its clinical significance must be established.

Lowering maintenance immune suppression in elderly kidney transplant recipients; connecting the immunological and clinical dots

The management of long-term immune suppressive medication in kidney transplant recipients is a poorly explored field in the area of transplant medicine. In particular, older recipients are at an increased risk for side effects and have an exponentially increased risk of infection-related death. In contrast, an aged immune system decreases the risk of acute T-cell-mediated rejection in older recipients. Recent advances in alloimmunity research have shown a rapid and substantial decline in polyfunctional, high-risk CD4⁺ T cells post-transplantation. This lowers the direct alloreactivity responsible for T-cell-mediated rejection, also known as donor-specific hyporesponsiveness. Chronic antibody-mediated rejection (c-aABMR) is the most frequent cause of kidney graft loss in the long term. However, in older adults, c-aABMR as a cause of graft loss is outnumbered by death with a functioning graft. In addition, DSA development and a diagnosis of c-aABMR plateau ~10 years after transplantation, resulting in a very low risk for rejection thereafter. The intensity of immune suppression regimes could likely be reduced accordingly, but trials in this area are scarce. Tacrolimus monotherapy for 1 year after transplantation seems feasible in older kidney transplant recipients with standard immunological risk, showing the expected benefits of fewer infections and better vaccination responses.

Natural Antibodies Are Associated With Rejection and Long-term Renal Allograft Loss in a Multicenter International Cohort

BACKGROUND

Potentially harmful nonhuman leukocyte antigen antibodies have been identified in renal transplantation, including natural immunoglobulin G antibodies (Nabs) reactive to varied antigenic structures, including apoptotic cells.

METHODS

In this retrospective, multicenter study, we assessed Nabs by reactivity to apoptotic cells in sera collected from 980 kidney transplant recipients across 4 centers to determine their association with graft outcomes.

RESULTS

Elevated pretransplant Nabs were associated with graft loss (hazard ratio [HR] 2.71; 95% confidence interval [CI], 1.15-6.39; P   =  0.0232), the composite endpoint of graft loss or severe graft dysfunction (HR 2.40; 95% CI, 1.13-5.10; P  = 0.0232), and T cell-mediated rejection (odds ratio [OR] 1.77; 95% CI, 1.07-3.02; P  = 0.0310). High pretransplant Nabs together with donor-specific antibodies (DSAs) were associated with increased risk of composite outcomes (HR 6.31; 95% CI, 1.81-22.0; P  = 0.0039). In patients with high pretransplant Nabs, the subsequent development of posttransplant Nabs was associated with both T cell-mediated rejection (OR 3.64; 95% CI, 1.61-8.36; P  = 0.0021) and mixed rejection (OR 3.10; 95% CI, 1.02-9.75; P  = 0.0473). Finally, elevated pre- and posttransplant Nabs combined with DSAs were associated with increased risk of composite outcomes (HR 3.97; 95% CI, 1.51-10.43; P  = 0.0052) and T cell-mediated rejection (OR 7.28; 95% CI, 2.16-25.96; P  = 0.0016).

CONCLUSIONS

The presence of pre- and posttransplant Nabs, together with DSAs, was associated with increased risk of poor graft outcomes and rejection after renal transplantation.

The Dual Function of RhoGDI2 in Immunity and Cancer

RhoGDI2 is a guanine nucleotide dissociation inhibitor (GDI) specific for the Rho family of small GTPases. It is highly expressed in hematopoietic cells but is also present in a large array of other cell types. RhoGDI2 has been implicated in multiple human cancers and immunity regulation, where it can display a dual role. Despite its involvement in various biological processes, we still do not have a clear understanding of its mechanistic functions. This review sheds a light on the dual opposite role of RhoGDI2 in cancer, highlights its underappreciated role in immunity and proposes ways to explain its intricate regulatory functions.

Sensitization in transplantation: Assessment of Risk 2022 Working Group Meeting Report

The Sensitization in Transplantation: Assessment of Risk workgroup is a collaborative effort of the American Society of Transplantation and the American Society of Histocompatibility and Immunogenetics that aims at providing recommendations for clinical testing, highlights gaps in current knowledge, and proposes areas for further research to enhance histocompatibility testing in support of solid organ transplantation. This report provides updates on topics discussed by the previous Sensitization in Transplantation: Assessment of Risk working groups and introduces 2 areas of exploration: non-human leukocyte antigen antibodies and utilization of human leukocyte antigen antibody testing measurement to evaluate the efficacy of antibody-removal therapies.

Association of HLA Mismatches and Histology Suggestive of Antibody-Mediated Injury in the Absence of Donor-Specific Anti-HLA Antibodies

BACKGROUND AND OBJECTIVES

The histology of antibody-mediated rejection after kidney transplantation is observed frequently in the absence of detectable donor-specific anti-HLA antibodies. Although there is an active interest in the role of non-HLA antibodies in this phenotype, it remains unknown whether HLA mismatches play an antibody-independent role in this phenotype of microcirculation inflammation.

DESIGN, SETTING, PARTICIPANTS, & MEASUREMENTS

To study this, we used the tools HLAMatchmaker, three-dimensional electrostatic mismatch score, HLA solvent accessible amino acid mismatches, and mismatched donor HLA-derived T cell epitope targets to determine the degree of HLA molecular mismatches in 893 kidney transplant recipients with available biopsy follow-up. Multivariable Cox proportional hazards models were applied to quantify the cause-specific hazard ratios of the different types of HLA mismatch scores for developing antibody-mediated rejection or histology of antibody-mediated rejection in the absence of donor-specific anti-HLA antibodies. In all survival analyses, the patients were censored at the time of the last biopsy.

RESULTS

In total, 121 (14%) patients developed histology of antibody-mediated rejection in the absence of donor-specific anti-HLA antibodies, of which 44 (36%) patients had concomitant T cell-mediated rejection. In multivariable Cox analysis, all different calculations of the degree of HLA mismatch associated with developing histology of antibody-mediated rejection in the absence of donor-specific anti-HLA antibodies. This association was dependent neither on the presence of missing self (potentially related to natural killer cell activation) nor on the formation of de novo HLA antibodies. Also, glomerulitis and complement C4d deposition in peritubular capillaries associated with the degree of HLA mismatch in the absence of anti-HLA antibodies.

CONCLUSIONS

The histology of antibody-mediated rejection and its defining lesions are also observed in patients without circulating anti-HLA antibodies and relate to the degree of HLA mismatch.

SARS-CoV-2 mRNA vaccination is not associated with the induction of anti-HLA or non-HLA antibodies

BACKGROUND

SARS-CoV-2 vaccination is strongly recommended in kidney transplant recipients (KTR) and dialysis patients. Whether these vaccinations may trigger alloantibodies, is still debated.

METHODS

In the current study we evaluated the effect of SARS-CoV-2 mRNA vaccines on anti-Human Leukocyte Antigen (HLA) and 60 anti-non-HLA antibody profiles in clinically stable KTR and dialysis patients. In total, we included 28 KTR, 30 patients on haemodialysis, 25 patients on peritoneal dialysis and 31 controls with a positive seroresponse 16-21 days after the first dose of either the SARS-CoV-2 mRNA BNT162b2 or mRNA-1273 vaccine. Both anti-HLA and anti-non-HLA antibodies were determined prior to vaccination and 21 to 35 days after the second vaccine dose.

RESULTS

Overall, the proportion of patients with detectable anti-HLA antibodies was similar before and after vaccination (class I 14% vs. 16%, p = 0.48; class II 25% before and after vaccination). After vaccination, there was no pattern in 1) additionally detected anti-HLA antibodies, or 2) the levels of pre-existing ones. Additional anti-non-HLA antibodies were detected in 30% of the patients, ranging from 1 to 5 new anti-non-HLA antibodies per patient. However, the clinical significance of anti-non-HLA antibodies is still a matter of debate. To date, only a significant association has been found for anti-non-HLA ARHGDIB antibodies and long-term kidney graft loss. No additionally developed anti-ARHGDIB antibodies or elevated level of existing anti-ARHGDIB antibodies was observed.

CONCLUSION

The current data indicate that SARS-CoV-2 mRNA vaccination does not induce anti-HLA or anti-non-HLA antibodies, corroborating the importance of vaccinating KTR and dialysis patients.

Non-HLA Antibodies in Kidney Transplantation: Immunity and Genetic Insights

The polymorphic human leukocyte antigen (HLA) system has been considered the main target for alloimmunity, but the non-HLA antibodies and autoimmunity have gained importance in kidney transplantation (KT). Apart from the endothelial injury, secondary self-antigen exposure and the presence of polymorphic alloantigens, respectively, auto- and allo- non-HLA antibodies shared common steps in their development, such as: antigen recognition via indirect pathway by recipient antigen presenting cells, autoreactive T cell activation, autoreactive B cell activation, T helper 17 cell differentiation, loss of self-tolerance and epitope spreading phenomena. Both alloimmunity and autoimmunity play a synergic role in the formation of non-HLA antibodies, and the emergence of transcriptomics and genome-wide evaluation techniques has led to important progress in understanding the mechanistic features. Among them, non-HLA mismatches between donors and recipients provide valuable information regarding the role of genetics in non-HLA antibody immunity and development.

Antigen and Cell-Based Assays for the Detection of Non-HLA Antibodies

To date, human leukocyte antigens (HLA) have been the major focus in the approach to acute and chronic antibody-mediated rejection (AMBR) in solid-organ transplantation. However, evidence from the clinic and published studies has shown that non-HLA antibodies, particularly anti-endothelial cell antibodies (AECAs), are found either in the context of AMBR or synergistically in the presence of donor-specific anti-HLA antibodies (DSA). Numerous studies have explored the influence of AECAs on clinical outcomes, yet the determination of the exact clinical relevance of non-HLA antibodies in organ transplantation is not fully established. This is due to highly heterogeneous study designs including differences in testing methods and outcome measures. Efforts to develop reliable and sensitive diagnostic non-HLA antibody tests are continuously made. This is essential considering the technical difficulties of non-HLA antibody assays and the large variation in reported incidences of antibodies. In addition, it is important to take donor specificity into account in order to draw clinically relevant conclusions from non-HLA antibody assays. Here, we provide an overview of non-HLA solid-phase and cell-based crossmatch assays for use in solid-organ transplantation that are currently available, either in a research setting or commercially.

The Pre-Transplant Non-HLA Antibody Burden Associates With the Development of Histology of Antibody-Mediated Rejection After Kidney Transplantation

Background

Many kidney allografts fail due to the occurrence of antibody-mediated rejection (ABMR), related to donor-specific anti-HLA antibodies (HLA-DSA). However, the histology of ABMR can also be observed in patients without HLA-DSA. While some non-HLA antibodies have been related to the histology of ABMR, it is not well known to what extent they contribute to kidney allograft injury. Here we aimed to investigate the role of 82 different non-HLA antibodies in the occurrence of histology of ABMR after kidney transplantation.

Methods

We included all patients who underwent kidney transplantation between 2004-2013 in a single center and had biobanked serum. Pre- and post-transplant sera (n=2870) were retrospectively tested for the presence of 82 different non-HLA antibodies using a prototype bead assay on Luminex (Immucor, Inc). A ratio was calculated between the measured MFI value and the cut-off MFI defined by the vendor for each non-HLA target.

Results

874 patients had available pretransplant sera and were included in this analysis. Of them, 133 (15.2%) received a repeat kidney allograft, and 100 (11.4%) had pretransplant HLA-DSA. In total, 204 (23.3%) patients developed histology of ABMR after kidney transplantation. In 79 patients (38.7%) the histology of ABMR was explained by pretransplant or de novo HLA-DSA. The multivariable Cox analysis revealed that only the broadly non-HLA sensitized (number of positive non-HLA antibodies) patients and those with the highest total strength of the non-HLA antibodies (total ratios of the positive non-HLA antibodies) were independently associated with increased rates of histology of ABMR after transplantation. Additionally, independent associations were found for antibodies against TUBB (HR=2.40; 95% CI 1.37 – 4.21, p=0.002), Collagen III (HR=1.67; 95% CI 1.08 – 2.58, p=0.02), VCL (HR=2.04; 95% CI 1.12 – 3.71, p=0.02) and STAT6 (HR=1.47; 95% CI 1.01 – 2.15, p=0.04). The overall posttransplant non-HLA autoreactivity was not associated with increased rates of ABMRh.

Conclusions

This study shows that patients highly and broadly sensitized against non-HLA targets are associated with an increased risk of ABMR histology after kidney transplantations in the absence of HLA-DSA. Also, some pretransplant non‐HLA autoantibodies are individually associated with increased rates of ABMR histology. However, whether these associations are clinically relevant and represent causality, warrants further studies.

Microvascular Inflammation of the Renal Allograft: A Reappraisal of the Underlying Mechanisms

Antibody-mediated rejection (ABMR) is associated with poor transplant outcomes and was identified as a leading cause of graft failure after kidney transplantation. Although the hallmark histological features of ABMR (ABMRh), i.e., microvascular inflammation (MVI), usually correlate with the presence of anti-human leukocyte antigen donor-specific antibodies (HLA-DSAs), it is increasingly recognized that kidney transplant recipients can develop ABMRh in the absence of HLA-DSAs. In fact, 40-60% of patients with overt MVI have no circulating HLA-DSAs, suggesting that other mechanisms could be involved. In this review, we provide an update on the current understanding of the different pathogenic processes underpinning MVI. These processes include both antibody-independent and antibody-dependent mechanisms of endothelial injury and ensuing MVI. Specific emphasis is placed on non-HLA antibodies, for which we discuss the ontogeny, putative targets, and mechanisms underlying endothelial toxicity in connection with their clinical impact. A better understanding of these emerging mechanisms of allograft injury and all the effector cells involved in these processes may provide important insights that pave the way for innovative diagnostic tools and highly tailored therapeutic strategies.

Antibody Responses to Minor Histocompatibility Antigens After Solid Organ Transplantation

Antibody-mediated rejection (AMR) is a major barrier to long-term graft survival following solid organ transplantation (SOT). Major histocompatibility antigens mismatched between donor and recipient are well-recognized targets of humoral alloimmunity in SOT and thought to drive most cases of AMR. In contrast, the implication of minor histocompatibility antigens (mHAs) in AMR has not been fully investigated, and their clinical relevance remains controversial. Recent technological advances, allowing for genome-wide comparisons between donors and recipients, have uncovered novel, polymorphic mHA targets with potential influence on the graft outcome following SOT. Here, we review these latest studies relating to mHAs and discuss their clinical significance.

Donor–Recipient Non-HLA Variants, Mismatches and Renal Allograft Outcomes: Evolving Paradigms

Despite significant improvement in the rates of acute allograft rejection, proportionate improvements in kidney allograft longevity have not been realized, and are a source of intense research efforts. Emerging translational data and natural history studies suggest a role for anti-donor immune mechanisms in a majority of cases of allograft loss without patient death, even when overt evidence of acute rejection is not identified. At the level of the donor and recipient genome, differences in highly polymorphic HLA genes are routinely evaluated between donor and recipient pairs as part of organ allocation process, and utilized for patient-tailored induction and maintenance immunosuppression. However, a growing body of data have characterized specific variants in donor and recipient genes, outside of HLA loci, that induce phenotypic changes in donor organs or the recipient immune system, impacting transplant outcomes. Newer mechanisms for “mismatches” in these non-HLA loci have also been proposed during donor–recipient genome interactions with transplantation. Here, we review important recent data evaluating the role of non-HLA genetic loci and genome-wide donor-recipient mismatches in kidney allograft outcomes.

Rho GTPases in kidney physiology and diseases

Rho family GTPases are molecular switches best known for their pivotal role in dynamic regulation of the actin cytoskeleton, but also of cellular morphology, motility, adhesion and proliferation. The prototypic members of this family (RhoA, Rac1 and Cdc42) also contribute to the normal kidney function and play important roles in the structure and function of various kidney cells including tubular epithelial cells, mesangial cells and podocytes. The kidney's vital filtration function depends on the structural integrity of the glomerulus, the proximal portion of the nephron. Within the glomerulus, the architecturally actin-based cytoskeleton podocyte forms the final cellular barrier to filtration. The glomerulus appears as a highly dynamic signalling hub that is capable of integrating intracellular cues from its individual structural components. Dynamic regulation of the podocyte cytoskeleton is required for efficient barrier function of the kidney. As master regulators of actin cytoskeletal dynamics, Rho GTPases are therefore of critical importance for sustained kidney barrier function. Dysregulated activities of the Rho GTPases and of their effectors are implicated in the pathogenesis of both hereditary and idiopathic forms of kidney diseases. Diabetic nephropathy is a progressive kidney disease that is caused by injury to kidney glomeruli. High glucose activates RhoA/Rho-kinase in mesangial cells, leading to excessive extracellular matrix production (glomerulosclerosis). This RhoA/Rho-kinase pathway also seems involved in the post-transplant hypertension frequently observed during treatment with calcineurin inhibitors, whereas Rac1 activation was observed in post-transplant ischaemic acute kidney injury.

Shotgun Immunoproteomics for Identification of Nonhuman Leukocyte Antigens Associated With Cellular Dysfunction in Heart Transplant Rejection

BACKGROUND

The International Society for Heart and Lung Transplant consensus panel notes that too little data exist regarding the role of non-HLA in allograft rejection. We developed a novel shotgun immunoproteomic approach to determine the identities and potential roles non-HLA play in antibody-mediated rejection (AMR) in heart transplant recipients.

METHODS

Serum was collected longitudinally from heart transplant recipients experiencing AMR in the absence of donor-specific anti-HLA antibodies (n = 6) and matched no rejection controls (n = 7). Antidonor heart affinity chromatography columns were formed by recipient immunoglobulin G immobilization at transplantation, acute rejection, and chronic postrejection time points. Affinity chromatography columns were used to capture antigens from individual patient's donor heart biopsies collected at transplantation. Captured proteins were subjected to quantitative proteomic analysis and the longitudinal response was calculated.

RESULTS

Overlap in antigen-specific response between AMR and non-AMR patients was only 8.3%. In AMR patients, a total of 155 non-HLAs were identified, with responses toward 43 high prevalence antigens found in ≥50% of patients. Immunofluorescence staining for representative high prevalence antigens demonstrated that their abundance increased at acute rejection, correlating with their respective non-HLA antibody response. Physiological changes in cardiomyocyte and endothelial cell function, following in vitro culture with patient immunoglobulin G, correlated with response toward several high prevalence antigens.

CONCLUSIONS

This work demonstrates a novel high-throughput strategy to identify clinically relevant non-HLA from donor endomyocardial biopsy. Such a technique has the potential to improve understanding of longitudinal timing of antigen-specific responses and their cause and effect relationship in graft rejection.

Allorecognition and the spectrum of kidney transplant rejection

Detection of mismatched human leukocyte antigens by adaptive immune cells is considered as the main cause of transplant rejection, leading to either T-cell mediated rejection or antibody-mediated rejection. This canonical view guided the successful development of immunosuppressive therapies and shaped the diagnostic Banff classification for kidney transplant rejection that is used in clinics worldwide. However, several observations have recently emerged that question this dichotomization between T-cell mediated rejection and antibody-mediated rejection, related to heterogeneity in the serology, histology, and prognosis of the rejection phenotypes. In parallel, novel insights were obtained concerning the dynamics of donor-specific anti-human leukocyte antigen antibodies, the immunogenicity of donor-recipient non-human leukocyte antigen mismatches, and the autoreactivity against self-antigens. Moreover, the potential of innate allorecognition was uncovered, as exemplified by natural killer cell-mediated microvascular inflammation through missing self, and by the emerging evidence on monocyte-driven allorecognition. In this review, we highlight the gaps in the current classification of rejection, provide an overview of the expanding insights into the mechanisms of allorecognition, and critically appraise how these could improve our understanding and clinical approach to kidney transplant rejection. We argue that consideration of the complex interplay of various allorecognition mechanisms can foster a more integrated view of kidney transplant rejection and can lead to improved risk stratification, targeted therapies, and better outcome after kidney transplantation.

Noninvasive Assessment of the Alloimmune Response in Kidney Transplantation

Transplantation remains the optimal mode of kidney replacement therapy, but unfortunately long-term graft survival after 1 year remains suboptimal. The main mechanism of chronic allograft injury is alloimmune, and current clinical monitoring of kidney transplants includes measuring serum creatinine, proteinuria, and immunosuppressive drug levels. The most important biomarker routinely monitored is human leukocyte antigen (HLA) donor-specific antibodies (DSAs) with the frequency based on underlying immunologic risk. HLA-DSA should be measured if there is graft dysfunction, immunosuppression minimization, or nonadherence. Antibody strength is semiquantitatively estimated as mean fluorescence intensity, with titration studies for equivocal cases and for following response to treatment. Determination of in vitro C1q or C3d positivity or HLA-DSA IgG subclass analysis remains of uncertain significance, but we do not recommend these for routine use. Current evidence does not support routine monitoring of non-HLA antibodies except anti-angiotensin II type 1 receptor antibodies when the phenotype is appropriate. The monitoring of both donor-derived cell-free DNA in blood or gene expression profiling of serum and/or urine may detect subclinical rejection, although mainly as a supplement and not as a replacement for biopsy. The optimal frequency and cost-effectiveness of using these noninvasive assays remain to be determined. We review the available literature and make recommendations.

Alemtuzumab as Second-Line Treatment for Late Antibody-Mediated Rejection of Transplanted Kidneys

Whether the anti-CD52 monoclonal antibody alemtuzumab can be an effective treatment option for late antibody-mediated rejection (ABMR) is not known. In a single-center pilot study, 12 patients with late ABMR were given 30 mg subcutaneous alemtuzumab.Median time from transplantation to biopsy was 22 months with 10 of 12 recipients fulfilling criteria for the histologic diagnosis chronic-active ABMR. The estimated glomerular filtration rate (eGFR) loss before diagnosis was 1.2 mL/min/mo with graft loss (eGFR <15 mL/min) expected to occur within 2 years in 11 of 12 cases. All recipients showed no or an inadequate response to initial treatment with steroids and intravenous immunoglobulin. eGFR at time of alemtuzumab administration was 35 mL/min/1.73 m² (IQR, 30-42) and stabilized or improved in 10 of 12 recipients within 12 months. Proteinuria was stable in the year after alemtuzumab. At 3-year follow-up, the death-censored graft survival was 68% (uncensored graft survival was 58%). Five cases of 10 cases that could be evaluated at 3-year follow-up had stable eGFR (on average 44 mL/min at 12 months and 42 mL/min at 36 months). Alemtuzumab was generally well tolerated and only 2 cases of opportunistic infections were noted. One case of symptomatic parvovirus B infection and 1 case of BK viral infection occurred, which both cleared at follow-up. In conclusion, alemtuzumab may be of value as a second-line treatment for late ABMR with rapid loss of eGFR.

Murine Cytomegalovirus-induced Complement-fixing Antibodies Deposit in Murine Renal Allografts During Acute Rejection

BACKGROUND

Human cytomegalovirus (CMV) infection is associated with renal allograft dysfunction and loss, particularly in combination with acute rejection. Emerging literature suggests that non-HLA antibodies may contribute to antibody-mediated rejection, but pathogen-induced antibodies have not been investigated in this context. This study examines the presence of CMV-induced antibodies in murine CMV (MCMV)-infected renal allografts during acute rejection.

METHODS

Intragraft immunoglobulin G (IgG) and complement C3 immunostaining were compared among allogeneic MCMV D-/R-, D+/R-, and D+/R+ renal transplants. Intragraft antibody deposition was examined in B cell-deficient recipients treated with MCMV immune sera. Antibody binding and complement-dependent cytotoxicity (CDC) of D-/R- and D+/R+ sera against infected renal tubular epithelial cells (TECs) were measured in vitro. IgG immunostaining was performed in D+/R+ allografts and native kidneys and in D+/R- allografts treated with ganciclovir to inhibit viral replication.

RESULTS

D+/R- and D+/R+ transplants had more abundant IgG and C3 deposition compared with D-/R- recipients. Greater IgG deposition was associated with more severe allograft injury in B cell-deficient recipients treated with MCMV immune sera compared with nonimmune sera. D+/R+ sera induced greater CDC of infected TECs compared with D-/R- sera. Native kidneys had lower IgG deposition compared with allografts, despite similar organ viral loads. Ganciclovir-treated allografts had reduced IgG deposition compared with untreated allografts.

CONCLUSIONS

In this murine model, complement-fixing antibodies can deposit into MCMV-infected renal allografts, are associated with allograft damage, and can induce CDC of MCMV-infected renal TECs. The allogeneic response and viral replication may also contribute to intragraft antibody deposition.

The Problem of Subclinical Antibody-mediated Rejection in Kidney Transplantation

Defined as histologic evidence of rejection on a protocol biopsy in the absence of kidney dysfunction, subclinical rejection has garnered attention since the 1990s. The major focus of much of this research, however, has been subclinical T cell-mediated rejection (TCMR). Herein, we review the literature on subclinical antibody-mediated rejection (AMR), which may occur with either preexisting donor-specific antibodies (DSA) or upon the development of de novo DSA (dnDSA). In both situations, subsequent kidney function and graft survival are compromised. Thus, we recommend protocol biopsy routinely within the first year with preexisting DSA and at the initial detection of dnDSA. In those with positive biopsies, baseline immunosuppression should be maximized, any associated TCMR treated, and adherence stressed, but it remains uncertain if antibody-reduction treatment should be initiated. Less invasive testing of blood for donor DNA or gene profiling may have a role in follow-up of those with negative initial biopsies. If a protocol biopsy is positive in the absence of detectable HLA-DSA, it also remains to be determined whether non-HLA-DSA should be screened for either in particular or on a genome-wide basis and how these patients should be treated. Randomized controlled trials are clearly needed.

A Comprehensive Overview of the Clinical Relevance and Treatment Options for Antibody-mediated Rejection Associated With Non-HLA Antibodies

Although solid organ transplant results have improved significantly in recent decades, a pivotal cause of impaired long-term outcome is the development of antibody-mediated rejection (AMR), a condition characterized by the presence of donor-specific antibodies to HLA or non-HLA antigens. Highly HLA-sensitized recipients are treated with desensitization protocols to rescue the transplantation. These and other therapies are also applied for the treatment of AMR. Therapeutic protocols include removal of antibodies, depletion of plasma and B cells, inhibition of the complement cascade, and suppression of the T-cell-dependent antibody response. As mounting evidence illustrates the importance of non-HLA antibodies in transplant outcome, there is a need to evaluate the efficacy of treatment protocols on non-HLA antibody levels and graft function. Many reviews have been recently published that provide an overview of the literature describing the association of non-HLA antibodies with rejection in transplantation, whereas an overview of the treatment options for non-HLA AMR is still lacking. In this review, we will therefore provide such an overview. Most reports showed positive effects of non-HLA antibody clearance on graft function. However, monitoring non-HLA antibody levels after treatment along with standardization of therapies is needed to optimally treat solid organ transplant recipients.

Histologic Antibody-Mediated Kidney Allograft Rejection in the Absence of Donor Specific HLA Antibodies

Histologic antibody-mediated rejection (hAMR) is defined as a kidney allograft biopsy satisfying the first 2 Banff criteria for diagnosing antibody-mediated rejection (AMR): tissue injury and evidence of current/recent antibody interaction with the endothelium. In approximately one-half of such cases, circulating HLA donor specific antibodies (DSA) are not detectable by current methodology at the time of biopsy. Some studies indicated a better prognosis for HLA-DSA-negative cases of hAMR compared to those with detectable HLA-DSA, whereas others found equally poor survival compared to hAMR-negative cases. We reviewed the literature regarding the pathophysiology of HLA-DSA-negative hAMR. We find 3 nonmutually exclusive possibilities: 1) HLA-DSA are involved, but just not detected; 2) non-HLA DSA (allo- or autoantibodies) are pathogenically involved; and/or 3) antibody-independent NK cell activation is mediating the process through "missing self" or other activating mechanisms. These possibilities are discussed in detail. Recommendations regarding the approach to such patients are made. Clearly, more research is necessary regarding the measurement of non-HLA antibodies, recipient/donor NK cell genotyping, and the use of antibody reduction therapy or other immunosuppression in any subset of patients with HLA-DSA-negative hAMR.

ARHGDIB and AT1R autoantibodies are differentially related to the development and presence of chronic antibody-mediated rejection and fibrosis in kidney allografts

The role of non-HLA autoantibodies in chronic-active antibody-mediated rejection (c-aABMR) of kidney transplants is largely unknown. In this study, the presence and clinical relevance of non-HLA autoantibodies using a recently developed multiplex Luminex-based assay were investigated. Patients with a kidney allograft biopsy at least 6 months after transplantation with a diagnosis of c-aABMR (n = 36) or no rejection (n = 21) were included. Pre-transplantation sera and sera at time of biopsy were tested for the presence of 14 relevant autoantibodies. A significantly higher signal for autoantibodies against Rho GDP-dissociation inhibitor 2 (ARHGDIB) was detected in recipients with c-aABMR as compared to recipients with no rejection. However, ARHGDIB autoantibodies did not associate with graft survival. Levels of autoantibodies against angiotensin II type 1-receptor (AT1R) and peroxisomal trans-2-enoyl-CoA reductase (PECR) were increased in recipients with interstitial fibrosis in their kidney biopsy. Only the signal for AT1R autoantibody showed a linear relationship with the degree of interstitial fibrosis and was associated with graft survival. In conclusion, anti-ARHGDIB autoantibodies are increased when c-aABMR is diagnosed but are not associated with graft survival, while higher levels of AT1R autoantibody are specifically associated with the presence of interstitial fibrosis and graft survival.

A Review on the Function and Regulation of ARHGDIB/RhoGDI2 Expression Including the Hypothetical Role of ARHGDIB/RhoGDI2 Autoantibodies in Kidney Transplantation

Challenging and still unsolved problems in kidney transplantation are risk stratification and the treatment of humoral rejection. Antibody-mediated rejection is an important cause of early and chronic rejection. The impact of donor-specific HLA antibodies on antibody-mediated rejection–causing graft damage is well known, but the clinical relevance of non-HLA antibodies remains unclear. Recently, in 2 independent studies, a new correlation was found between the presence of non-HLA anti-Rho guanosine diphosphate dissociation inhibitor 2 (ARHGDIB) antibodies and increased graft failure. RhoGDI2, another name for ARHGDIB, is a negative regulator of the Rho guanosine triphosphate (RhoGTP)ases RhoA, Rac1m, and Cdc42, whose main function is regulating the actin network in a variety of cells. RhoGDI2 is mainly expressed intracellularly, and some expression is observed on the cell surface. Currently, there is no mechanism known to explain this correlation. Additionally, the reason why the antibodies are produced is unknown. In this review, we will address these questions, provide an overview of other diseases in which these antibodies are prevalent, and describe the physiological role of RhoGDI2 itself. If the mechanism and impact of RhoGDI2 antibodies in kidney graft failure are known, improved risk stratification can be provided to decrease the rate of donor kidney graft failure.